YARN and YARN Applications

YARN applications are somewhere where Hadoop authentication becomes some of its most complex.

Anyone writing a YARN application will encounter Hadoop security, and will end up spending time debugging the problems. This is "the price of security".

YARN Service security

YARN Resource Managers (RM) and Node Managers (NM) perform work on behalf of the user.

The NM's

  1. Localize resources: Download from HDFS or other filesystem into a local directory. This is done using the delegation tokens attached to the container launch context. (For non-HDFS resources, using other credentials such as object store login details in cluster configuratio files)

  2. Start the application as the user.

Securing YARN Application Web UIs and REST APIs

YARN provides a straightforward way of giving every YARN application SPNEGO authenticated web pages: it implements SPNEGO authentication in the Resource Manager Proxy. YARN web UI are expected to load the AM proxy filter when setting up its web UI; this filter will redirect all HTTP Requests coming from any host other than the RM Proxy hosts to an RM proxy, to which the client app/browser must re-issue the request. The client will authenticate against the principal of the RM PRoxy (usually yarn), and, once authenticated, have its request forwared.

As a result, all client interactions are SPNEGO-authenticated, without the YARN application itself needing any kerberos principal for the clients to authenticate against.

Known weaknesses in this approach are

  1. As calls coming from the proxy hosts are not redirected, any application running on those hosts has unrestricted access to the YARN applications. This is why in a secure cluster the proxy hosts must run on cluster nodes which do not run end user code (i.e. not run YARN NodeManagers and hence schedule YARN containers).
  2. The HTTP requests between proxy and YARN RM Server are not encrypted.

Securing YARN Application REST APIs

YARN REST APIs running on the same port as the registered web UI of a YARN application are automatically authenticated via SPNEGO authentication in the RM proxy.

Any REST endpoint (and equally, any web UI) brought up on a different port does not support SPNEGO authentication unless implemented in the YARN application itself.

Strategies for token renewal on YARN services

Keytabs for AM and containers

A keytab is provided for the application. This can be done by:

  1. Installing it in every cluster node, then providing the path to this in a configuration directory. The keytab must be in a secure directory path, where only the service (and other trusted accounts) can read it.

  2. Including the keytab as a resource for the container, relying on the Node Manager localizer to download it from HDFS and store it locally. This avoids the administration task of installing keytabs for specific services. It does require the client to have access to the keytab and as it is uploaded to the distributed filesystem, must be secured through the appropriate path permissions.

This is the strategy adopted by Apache Slider (incubating). Slider also pushes out specified keytabs for deployed applications such as HBase, with the Application Master declaring the HDFS paths to them in its Container Launch Requests.

AM keytab + renewal and forwarding of Delegation Tokens to containers

The Application Master is given the path to a keytab (usually a client-uploaded localized resource), so can stay authenticated with Kerberos. Launched containers are only given delegation tokens. Before a delegation token is due to expire, the processes running in the containers must request new tokens from the Application Master. Obviously, communications between containers and their Application Master must themselves be authenticated, so as to prevent malicious code from requesting the containers from an Application Master.

This is the strategy used by Spark 1.5+. Communications between container processes and the AM is over HTTPS-authenticated Akka channels.

Client-side push of renewed Delegation Tokens

This strategy may be the sole one acceptable to a strict operations team: a client process running on an account holding a kerberos TGT negotiates with all needed cluster services for delegation tokens, tokens which are then pushed out to the Application Master via some RPC interface.

This does require the client process to be re-executed on a regular basis; a cron or OOzie job can do this.

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